A CDMA cellular telephone performs a handoff as the cellular telephone moves from one cell to another, in order to switch the communication channel from a currently communicating base station (also referred to as “previous base station”) to a destination base station. There are two types of handoffs: a soft handoff and a hard handoff. When performing the soft handoff, the cellular telephone starts communicating with the destination base station using the same frequency as the frequency currently used, before closing the communication with the communicating base station. When performing the hard handoff, the cellular telephone starts a communication, after closing the communication with the communicating base station, with the destination base station in a different base station group, having a different frequency allocation, or having a different frame offset.
The soft handoff is performed based on pilot intensities of neighboring base stations. A pilot intensity is an intensity of a pilot channel common to the neighboring base stations, but outputted at a different spread timing from each base station, and also indicates a ratio of reception pilot energy in an entire reception energy. The cellular telephone measures the pilot intensities of the neighboring base stations on a regular basis, and notifies the communicating base station of the measured intensities. If any of the neighboring base stations is noted to have a sufficient pilot intensity that is stronger than the communicating base station, the communicating base station instructs the cellular telephone to perform a soft handoff to switch to the noted neighboring base station. Because the cellular telephone has already specified the spread timing (a synchronization timing) of all of the neighboring base stations by measuring the pilot intensities, the cellular telephone can synchronize with the noted neighboring base station as soon as the soft handoff instruction is received.
On the other hand, the hard handoff is for switching base stations that are in different base station groups or have different frequency allocations or different frame offsets, and a hardware structure does not allow the cellular telephone to measure a pilot intensity of the destination base station before actually performing the handoff. Therefore, the cellular telephone measures the pilot intensity of the destination base station only after the hard handoff instruction is received from the communicating base station, and specifies the synchronization timing of the destination base station based on the measurement. When measuring the pilot intensity of the destination base station, the cellular telephone estimates the synchronization timing of the destination base station based on the synchronization timing of the previous base station, and then sets a search window in accordance with the estimated synchronization timing and measures the pilot intensity during a period that the search window indicates.
As has been described, the search window in the hard handoff is set based on the synchronization timing of the previous base station. Accordingly, when radiowaves from the destination base station delay considerably due to reflections and such, a pilot signal peak of the destination base station falls outside the search window and the cellular telephone fails to acquire the synchronization timing of the destination base station. Such a failure of the handoff would cease a telephone call.